FIG. 8 is a circuit diagram showing a write current driving circuit based on the conventional technology. This write current driving circuit comprises two NPN transistors T1, T2 on the current-supply side; two NPN transistors T3, T4 on the current-extraction side; a write head WH made up of a coil; two NPN transistors T5, T6 for biasing a base potential of the transistors T1, T2; and two loading resistors R1, R2.
Collectors of the transistors T1 and T2 are connected to a power supply unit Vcc. Bases of the transistors T1 and T2 are connected to the power supply unit Vcc through the resistors R1 and R2 respectively. Emitters of the transistors T1 and T2 are connected to the collectors of the transistors T3 and T4 respectively.
Signals IN1 and IN2 inputted from external devices are supplied as base potentials to the bases of the transistors T3 and T4 respectively. The input signal IN2 is an inverted signal of the other input signal IN1. Emitters of the transistors T3 and T4 are commonly connected to a current source 11, thus a current flows from the emitters of the transistors T3 and T4 into the current source 11.
Collectors of the transistors T5 and T6 are connected to the bases of the transistors T1 and T2 respectively. Base potentials are fed by the signals IN1 and IN2 to the bases of the transistors T5 and T6 respectively. Emitters of the transistors T5 and T6 are commonly connected to a current source 12, thus a current flows from the emitters of the transistors T5 and T6 into the current source 12.
One end of the coil of the write head WH is connected to the emitter of the transistor T1 and the collector of the transistor T3. The other end is connected to the emitter of the transistor T2 and to the collector of the transistor T4.
Effects of the write current driving circuit configured as shown in FIG. 8 will be described below. When one input signal IN1 is at a relatively high potential level (described as HIGH hereinafter) and the other input signal IN2 is at a relatively low potential level (described as LOW hereinafter), the transistors T3 and T5 are ON, while the transistors T4 and T6 are OFF.
As the transistor T5 is ON, a current passes through the loading resistor R1. With that current, a base potential of the transistor T1 drops and the transistor T1 is turned OFF. On the other hand, as the transistor T6 is OFF, a current does not pass through the loading resistor R2. Accordingly, a base potential of the transistor T2 is maintained at the power supply potential as it is, and so the transistor T2 is turned ON. Therefore, an emitter current of the transistor T2 passes through the write head WH from the side of the transistor T2 toward the side of the transistor T3, and becomes a collector current of the transistor T3.
When the input signal IN1 is LOW and the other input signal IN2 is HIGH, the transistors T3 and T5 are OFF while the transistors T4 and T6 are ON. Accordingly, a current passes through the loading resistor R2, and with that current, a base potential of the transistor T2 drops, and the transistor T2 is turned OFF. On the other hand, as the transistor T5 is OFF, a current does not pass through the loading resistor R1, and so a base potential of the transistor T1 is maintained at the power supply potential as it is, and the transistor T1 is turned ON.
Therefore, an emitter current of the transistor T1 passes through the write head WH from the side of the transistor T1 toward the side of the transistor T4, and becomes a collector current of the transistor T4. As described above, by inputting a pair of inverted signals IN1 and IN2, a current passing through the write head WH, namely a direction of a write current to a disk is switched.
In recent years, it has been desired that data is written in a hard disk or the like with higher speed. To increase the writing speed, it is required to increase the switching speed when a direction of a write current is switched and also to speed up a rise time (Tr) and a fall time (Tf) of the current on switching.
In the conventional type of write current driving circuit shown in FIG. 8, however, there are base-emitter capacitor and base-collector capacitor in input sections of the transistors T1 and T2, and the capacities form a low-pass filter together with the loading resistors R1 and R2. The formed filter makes frequency response worse, and so the rise time and the fall time of a write current when the direction of the write current is switched becomes longer. If resistances of the loading resistors R1 and R2 are made smaller to overcome the problem, a current required to supply a base voltage to the transistors T1 and T2 becomes larger, which requires larger power consumption.